Promise.method() function not working the way I expect it to work

Question

I am writing some code that encrypts a communication channel between two users. The steps are as follows

• Check if a cipher for the channel exists in mongoDb
• If yes, fetch the cipher and encrypt the incoming data
• If no, create a new cipher, save it to mongoDb and encrypt the incoming data

The code uses crypto module which is one of the few synchronous libs in core nodejs

Create a cipher and return it as a promise

cipher.createCipher = Promise.method((pw) => {
  if (!pw) {
    throw new Error('Passphrase must be provided');
  }
  return crypto.createCipher('aes192', pw);
});

Encrypt data using Promise.method()

cipher.encryptTextAsPromise = Promise.method((cipher, plainText) => {
  if (!( typeof plainText === 'string')) {
    throw new Error("2nd param must be plain text");
    let cipherText = '';
    cipher.on('readable', () => {
      var data = cipher.read();
      if (data)
        cipherText += data.toString('hex');
    });
    cipher.on('end', () => {
      return cipherText;
    });
    cipher.write(plainText);
    cipher.end();
  }
});

Encrypt data via callback functions.

cipher.encryptText = (cipher, plainText, callback) => {
  if (!( typeof plainText === 'string')) {
    throw new Error("2nd param must be plain text");
  }
  try {
    let cipherText = '';
    cipher.on('readable', () => {
      var data = cipher.read();
      if (data)
        cipherText += data.toString('hex');
    });
    cipher.on('end', () => {
      callback(null, cipherText);
    });
    cipher.write(plainText);
    cipher.end();
  } catch (e) {
    callback(e, null);
  }
}

I am unable to chain these two together. What I have is a horrible anti-pattern that is worse than getting stuck in callback hell

cipher.createCipher('secretWord')
  .then((data) => {
    cipher.encryptTextasPromise(data, 'hello world')
    .then((data) => {
      console.log(data);
    })
    .catch((err) => {
      console.log(err);
    })
  })
  .catch((err) => {
    console.log(err);
  })

mongoDbSearch(username)
  .then((data) => {
    if (data) {
      // Cipher exists, retrieve and encrypt
    }else {
      // Create new cipher and save
      someMongoSave()
        .then((data) => {
          // Cipher exists now, retrieve and encrypt
        })
    }
  })
  .catch((mongoErr) => {
    console.log(mongoErr);
  })

The code is a bit incomplete as I am still trying to grasp this concept. Furthermore the my attempt to chain createCipher and encryptTextAsPromise is returning undefined in the data. I have tried writing these as normal callback and then using Promise.promisfyAll() as well, which just feels like another anti-pattern.

Answer 1

Promise.method would appear to make sense for createCipher, but probably not for encryptTextAsPromise.

Here's a version that should point you the right direction, distilling various of T.J. Crowder's, Jaromanda X's, and undefined's comments on the question; see code comments for more:

// For this one, `Promise.method` still makes sense (although doing it with
// your own promise is also perfectly reasonable)
cipher.createCipher = Promise.method(pw => {
  if (!pw) {
    throw new Error('Passphrase must be provided');
  }
  return crypto.createCipher('aes192', pw);
});

// For this one, your own promise makes sense
cipher.encryptTextAsPromise = (cipher, plainText) => {
  return new Promise(resolve => {
    if (!( typeof plainText === 'string')) {
      // It's fine to throw here (it will get converted into a rejection),
      // or add `reject` to the arguments list above and call
      // that and return instead:
      // reject(new Error("2nd param must be plain text"));
      // return;
      throw new Error("2nd param must be plain text");
    }
    let cipherText = '';
    cipher.on('readable', () => {
      var data = cipher.read();
      if (data)
        cipherText += data.toString('hex');
    });
    cipher.on('end', () => {
      resolve(cipherText);
    });
    cipher.write(plainText);
    cipher.end();
  });
};

// Usage:
cipher.createCipher('secretWord')
.then(data => cipher.encryptTextAsPromise(data, 'hello world'))
.then(data => console.log(data)) // See 1 below
.catch(err => console.log(err));

mongoDbSearch(username)
.then(data => data || someMongoSave(data)) // You pass `data` to `someMongoSave`, presumably?
.then(data => {
    // Cipher exists, retrieve and encrypt
})
.catch(mongoErr => {
    console.log(mongoErr);
});

Separately, re

if (!( typeof plainText === 'string')) {

there's a !== operator in JavaScript. Jus' sayin'. ;-D